A frame transfer CCD imager generally includes three parts: a photosensing array, known as the A-register; a temporary storage array, known as the B-register; and an output register, known as the C-register. Each of the A-register and the B-register includes a plurality of parallel channel regions in a substrate of single crystalline silicon and extending along a surface of the substrate. The channel regions of the B-register are extensions of the channel regions of the A-register. Between the channel regions are channel stop regions which isolate the channels from each other. As described by L. F. Wallace in U.S. Pat. No. 4,362,575, issued Dec. 7, 1982, entitled "Method of Making Buried Channel Charge-Coupled Device With Means for Controlling Excess Charge", a blooming drain can be provided between the channels of the A-register to collect any excess charge from the channels. Each of the A-register and B-register has a plurality of parallel conductive gates extending transversely across the channels and insulated from the substrate by a layer of an insulating material, such as silicon oxide. The C-register includes a single channel region in the substrate extending transversely across the ends of the B-register channels. A plurality of parallel conductive gates extends transversely across the C-register channel in a direction substantially parallel to the B-register channels. The C-register gates are insulated from the substrate surface by a layer of insulating material, such as silicon oxide.
In each of the registers the gates are arranged in sets with the common gates of each set being electrically connected together. Thus, the first gates of each set are connected together, the second gates of each set are connected together, etc. The number of gates in each set depends on the number of phases of the CCD. For a three phase CCD there are three gates per set, a four phase CCD has four gates per set, etc. The common gates are either connected together by a conductive termination strip or extend to conductive termination pads, which are part of the conductive layer which forms the gates, and which have contacts to which the termination wires can be connected. In the A-register and B-register the termination strips or pads are generally along one side of the register array. In the C-register the termination strips or pads are along the side of the channel away from the B-register. Also, the gates are generally formed from one or more layers of the conductive material, such as conductive polycrystalline silicon. For a two or four phase CCD, the gates are generally formed from two layers of the conductive material with at least one sets of gates being formed from each of the layers which are insulated from each other by a layer of silicon oxide.
The trend in semiconductor integrated circuits, including CCD imagers, is to make the device denser using narrower conductive lines and smaller related regions in the semiconductor substrate. For a CCD imager this allows an imager having a given size array to be made on a smaller substrate, or an imager having a larger array to be formed on the same size substrate. However, making the CCD imager denser raises a problem of where to place the termination strips or pads since the termination strips or pads must be relatively larger than the gates to allow terminal wires to be connected thereto. In the A-register and B-register, this problem can be handled by arranging some of the termination strips or pads along one side of the array and the others along the other side of the array. However, the problem still exists in the C-register which has only one side along which the termination strips or pads can be easily arranged.
Another problem which arises in making denser CCD imagers is in making the electrical connection between the buried channel regions of the B-register and the buried channel region of the C-register. This connection is generally achieved by having each of the B-register buried channel regions extend under the C-register gates so that a potental applied to the C-register gates will transfer charge from the B-register channel region to the C-register channel region. In order to achieve this transfer with good efficiency, the end of the B-register buried channel region should extend completely under the C-register gates. However, in a dense imager, the B-register buried channel may not extend completely under the C-register gates leaving some portions exposed, i.e. not covered. This can result from either the difference in width of the B-register channels, which are in the order of 8 micrometers in width, and the C channel gates, which are in the order of 4 micrometers in width, or by misalignment of the masks used to form the channels and gates. The exposed portions of the B-register buried channels will provide wells which can trap charges in the B-register channels and prevent such charges from being transferred to the C-register channel. Thus, it is necessary to align as much of the B-register channel regions as possible with the C-register gates to eliminate any exposed portions of the B-register channel region which can form charge trapping pockets.